The electron's mass is about 9×10^-28g, 1,836 times less than that of the proton. Electrons orbit the positively charged nuclei of atoms and are responsible for binding atoms together in molecules, as well as for the electrical, thermal, optical, and magnetic properties of solids. Electric currents in metals and in semiconductors consist of a flow of electrons, and light, radio waves, X-rays, and much heat radiation are all produced by accelerating and decelerating electrons.